Copy producing materials which exploit the photosensitivity of diazo compounds are widely utilized because of their inexpensiveness. These copy producing materials are generally developed according to one of three major types of development, which are as follows:
The first type is known as the wet development type, where the copy producing material is characterized by a photosensitive layer mainly comprising a diazo compound and a coupling component on a support, and it is developed by an alkaline solution after exposure to light together with an original manuscript superposed on the top thereof.
The second type is known as the dry development type, where the exposed photosensitive layer is developed by ammonia gas instead of an alkaline solution per the wet development type.
The third type is known as the thermal development type, where the photosensitive layer contains an ammonia gas generating agent, such as urea, that generates ammonia gas upon heating, or, alternatively, it contains an alkaline salt compound, such as trichloroacetic acid, that loses its acidic properties upon heating or it exploits activation of a diazo compound and a coupling component through thermal fusion by adding a higher fatty acid amide as a color-forming aid.
The above wet type of development has various drawbacks. For example, the use of the alkaline developing solution takes time for refilling and disposing thereof, and the large size of the copying devices creates difficulties in maintenance and control. Further, entering additional exposures and images on the wet recording material is practically impossible immediately after copy-making when the material is still wet, and the copied images cannot endure long storage.
The dry type of development has several drawbacks as well. For instance, refilling of the developing solution is necessary, and an ammonia gas suction device should be provided to prevent leakage of the generated gas, which enlarges the size of the copying device, and a strong odor of the noxious ammonia is unavoidable immediately after copy-producing.
Apart from the wet type or the dry type of development, the thermal developing type has merit in terms of lessened maintenance as it does not require the use of a developing solution. However, it still has a shortcoming in that the copy forming apparatus becomes expensive in order to obtain satisfactory images, as the development temperature is as high as 150.degree. to 200.degree. C. and thermal controls must be made within the range of .+-.10.degree. C. to prevent variations in color tone due to inadequate development.
To stand such high temperatures required in thermal development, the diazo compounds to be used must be heat-resisting, but these compounds are disadvantageous to form images of high density.
Therefore, many attempts in the past have been attempted to make developments at low temperature (e.g., 90.degree. to 130.degree. C.), but it had a defect of shortening the shelf life of the copy material. The current situation is that the thermal development type of development is not yet occupying a position as the main stream in the diazo copying system, in spite of the prediction that the thermal development type has advantages in terms of lessened maintenance in comparison to the wet and dry types of development.
On the other hand, the needs of users may be completely diverse. For example, needs are arising not only to obtain colored images on a white background, as was the case in the past, but also to select the background and a hue of the image in correspondence with the use desired.
This need arises because copy images should draw the viewer's attention when such images are used as drawings or notices. However, the existing recording materials could not fully satisfy such visual requirements.
Incidentally, in order to obtain the desired color density by heating a recording material provided with a layer containing a diazo compound, a coupling component and a color-forming aid, then the respective constituents, as mentioned above, should be able to melt, disperse and react to form the coloring matter in an instantaneous manner.
However, if the recording material was designed so as to well form the color and produce images of high density at the low heating temperatures, then the color-forming reaction would tend to gradually proceed inadvertently during storage at normal room temperature before usage, and, as a consequence, the background of the recording material to be non-exposed and kept white would be colored and thus spoiled. That is, background stains occur in the recording layer before recording.
The above-described problem was somewhat overcome by a proposal for enclosing a diazo compound in microcapsules in the copy-making material provided with a photosensitive layer, which contains all of the diazo compound, a coupling constituent and a color-forming aid, on the support, and which can be thermally developed (e.g., JP-A-59-91438). The term "JP-A" as used herein means an "unexamined published Japanese patent application".
However, this case still had a drawback that preservability of the recording material before use is insufficient when the photosensitive layer is designed to obtain images of very high density.